Enteromorpha prolifera biochar as a novel ball milling aid for enhancing the interfacial reaction activity of microscale zero-valent iron (mZVI) for Cr(VI) removal from water

Abstract

Ball milling (BM) is a promising approach to activate microscale zero-valent iron (mZVI), in which the grinding aids are essential. Yet, previous researches have shown that carbon could act as a grinding aid while the grinding mechanism of carbon is seldom revealed. In this study, Enteromorpha prolifera biochar (EBC), from the pyrolysis of large harmful algae, was selected as a novel grinding aid to prepare a carbon-modified mZVI composite (EBC-mZVIbm) to enhance the activity of mZVI in water. After a simple BM process with the help of carbon, the previously inert mZVI exhibited excellent reductive performance. EBC outperformed other carbons as grinding aid for Cr(VI) removal over a wide pH range (3.00–11.00), and EBC dosage and rotation speed were the critical factors for the mZVI activating in the BM process. The grinding mechanism of EBC was demonstrated. EBC could diffuse to the mZVI surface under mechanical collision and serve as a surface coating to inhibit the agglomeration of mZVI. The presence of EBC enhanced the electrical conductivity and adsorption ability by changing the specific surface area, the oxygen-containing functional groups and defective structures, which facilitated the interfacial reaction between mZVI and Cr(VI). Cr(VI) was removed by a chemisorption-reduction process and Fe(II) played a key role. There were no chromium and iron ions in the solution after reaction. This study provided a new strategy for efficiently activating mZVI with a dry one-pot procedure, a new understanding of carbon grinding mechanism and a new material for heavy metal ions recovery.